1. Field of the Invention
The invention relates to the field of transition sensors, and in particular to a sensing device for sensing, indicating and responding to a transition of a positive temperature coefficient resistive element into a state of high resistance.
2. Prior Art
Certain materials are known in the art to be useful as reusable fuses for limiting electrical current. These materials, known as positive temperature coefficient (PTC) resistive elements, respond to a flow of excessive current by substantially increasing in electrical resistance due to resistive heating. For example, as stated in PTC Application Notes, Keystone Carbon Company Bulletin T-929, P.37, "[t]he dramatic rise in resistance of a PTC at the transition temperature makes it an ideal candidate for current limiting applications. For currents below the limiting current (I.sub.L), the power being generated in the unit is insufficient to heat the PTC to its transition temperature. However, when abnormally high-fault currents flow, the resistance of the PTC increases at such a rapid rate that any further increase in power dissipation results in a reduction in current."
The PTC's manufactured and marketed by Keystone Carbon Company are made of polycrystalline ceramic materials. The base compounds, usually barium titanate or solid solutions of barium and strontium titanate, are high-resistivity materials made semiconductive by the addition of dopants.
Conductive polymers also exhibit PTC characteristics. For example, Raychem Corporation manufactures and markets a current limiting polymer under the trademark Polyswitch.TM.. Current limiting polymers having PTC characteristics are disclosed in U.S. Pat. Nos. 4,545,426; 4,560,498 and 4,775,778, owned by Raychem Corporation. Current limiting polymers typically comprise crosslinked polyethylene, heavily doped with carbon. Other materials exhibiting PTC resistance characteristics include silicon carbide and tungsten.
PTC elements for current limiting applications typically have a low electrical resistance when conducting current below a threshold value, i.e., when the PTC is relatively cool. When current flowing through the PTC exceeds the threshold, resistive heating produces a rise in the internal temperature of the PTC, causing a reduction in conductivity, i.e., an increase in electrical resistance. The power dissipated in the PTC is proportional to the resistance multiplied by the square of the current. An increase in resistance leads to further heating and a further increase of resistance. The change in resistance thus is quite rapid. Typically, the increase in resistance is virtually a step function once the magnitude of the current (and the resulting internal temperature of the polymer) surpasses the threshold value.
The change in resistance of a PTC upon passing the threshold can be quite large. For example, the resistance of a current limiting polymer upon passing the threshold may increase by a factor of 1,000 to 4,000. Assuming the PTC is in a power distribution circuit in series with a load, the increase in resistance of the PTC increases the total load resistance seen by the power line and substantially reduces the current drawn from the line. The increase in resistance of the PTC, in series with a fixed resistance load, results in more of the power line voltage occurring as a voltage drop across the PTC and less in the voltage drop across the load. Thus, a larger portion of the power from the line is dissipated in the PTC as heat, as opposed to being dissipated by the load. Depending on the line voltage, load resistance and character of the PTC element, the voltage drop across a PTC which has transitioned to its high resistance state could be large enough to destroy the PTC. This is especially true when the PTC comprises a conductive polymer. Furthermore, PTCs may-exhibit-a negative temperature coefficient (NTC) resistance characteristic if the internal temperature of the PTC goes much beyond the threshold level. If heated to the NTC level, the resistance of the PTC decreases with increasing temperature.
A PTC resistive element employed as a reusable fuse typically is coupled in series between a power source and a load for limiting current upon transition to its high resistance state. A short circuit, wiring fault or the like may cause the load circuit to sink excessive current. The PTC resistive element will conduct this excessive current for only the relatively short time until the PTC element makes a step transition into its state of high resistance. The current loading of the power line is substantially reduced; however, prolonged excessive power dissipation in the PTC resistive element could damage the PTC and require its subsequent replacement. If the PTC element is to be truly reusable, action is needed to decouple the PTC element and the load circuit from the power line before the PTC element is damaged. Furthermore, it is desirable that an operator, technician or the like be quickly advised of a circuit fault, i.e., that the PTC element has transitioned to a high resistance state, since this is symptomatic of a substantial problem in the load or power delivery circuit. It may also be appropriate in the event of a fault to decouple other related circuits from the power source. There is a need, therefore, to provide means for sensing that a current limiting PTC resistive element has transitioned to its state of high resistance. Advantageously, this sensing means is arranged for signalling and for automatically decoupling power from a malfunctioned load circuit.